The present invention relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to methods and apparatus for simultaneously receiving a different NMR response signal from each of a plurality of closely-positioned radi-frequency (RF) coils, having substantially reduced interactions therebetween.
Present-day NMR imaging systems utilize receiver coils which surround the entire sample (for example, a human patient) which is to be imaged. These "remote coils" have the advantage that the sensitivity to individual spins is, to a first approximation, substantially constant over the entire region being imaged. Although this uniformity is not strictly characteristic of such remote coils, it is substantially constant to a sufficient degree that most present-day reconstruction techniques assume a constant coil sensitivity. Because of their large size, such remote coils suffer from two disadvantages: first, a relative insensitivity to individual spins; and, second, a relatively large inductance and, therefore, a low self-resonant frequency. It is well known that surface coils do not have a uniform sensitivity to individual spins within the region; images produced using surface coils requie additional compensation for such inhomogeneity. Surface coils can, however, be made much smaller in geometry than remote coils and for medical diagnostic use can be applied near, or on, the body surface of the sample patient. This is especially important where attention is being directed to imaging a small region within the sample, rather than an entire anatomical cross section. Because the surface coil reception element can be located closer to the spins of interest, a given spin will produce a larger EMF, at a given Larmor frequency, in a surface coil than in a remote coil. The use of a surface coil also reduces the nose contribution from electrical losses in the body, with respect to a corresponding remote coil, while maximizing the desired signal.
NMR imaging systems thus typically use a small surface coil for localized high resolution imaging. A single surface coil of diameter D gives the highest possible single-to-noise ratio (SNR) for that volume around a depth D inside an infinite conducting half space. However, the single surface coil can only effectively image that region with lateral dimensions comparable to the surface coil diameter D. Therefore, the use of a surface coil necessarily restricts the field-of-view and inevitably leads to a trade-off between resolution and field-of-view. Since the fundamental limitation of the SNR of a surface coil is its intrinsic signal-to-noise ratio, wherein the noise resistance is attributable to currents induced in the sample (for example, a patient in a medical NMR imaging situation) by the radio-frequency (RF) receiving coil. Larger coils induce greater patient sample losses and therfore have a larger noise resistance; smaller coils have a lower noise resistance but, in turn, restrict the field of view to a smaller region.
It is highly desirable to extend the field-of-view by providing a set of surface coils arrayed with overlapping fields-of-view. However, it is desirable to at the same time maintain the high SNR of the single surface coil, if at all possible; this requires that mutual coil interactions be alleviated. It is also highly desirable to be able to construct a single optimal image, which maximizes the signal-to-noise ratio in each pixel of the composite single image, from the partial-image data of each of the plurality of surface coils in the array.